1. Field of the Invention
The present invention relates to an electric linear stepper motor comprising a multipolar stator with a threaded bore, a rotor having a diameter less than that of said bore, threaded to the same pitch as said bore and rolling cycloidally thereinside under the action of stator forces, and a drive rod fixed axially to said rotor and thus driven in linear translation.
Such a motor is used, for example, in precision regulation systems. In such systems, the drive rod which moves linearly and step by step, controls regulation members. It is often called control rod.
2. Description of the Prior Art
A motor of this type is already known from the European patent no. 0 078 740. The threads of the rotor are very often parallel grooves, having the same pitch as the threads of the stator, which are then helical and obtained by tapping the bore of the stator. The rotational movement of the rotor is thus transformed into a linear movement.
When the coils of the stator are energized successively, under the action of the stator forces the rotor rotates by angular steps whose number, through 360.degree. is defined by the number of poles of the stator. Thus, an axial advance equal to the pitch of the stator corresponds to each revolution of the rotor.
When none of the coils of the stator is energized, the rotor is no longer attracted by the stator, it is in what is called a disengaged situation and may be brought back, by adapted return means, to one of the endmost positions of the range of axial positions which it may occupy relatively to the stator. This endmost position then forms its rest position.
In this type of motor, since the rotor is driven with a cycloidal movement, the drive rod or control rod is driven with a movement which causes its axis to describe a circular cylinder. Since the member to be controlled is generally not capable of adapting itself to this movement, the control rod is connected to the member to be controlled by a double articulation, or it comprises a flexible intermediate portion. These solutions have the drawbacks of lacking in precision, of being complex and expensive and, in the case where a flexible intermediate portion is used, of only allowing a single operating direction, the one for which this portion is under traction. Another drawback of this type of motor is that the disengagement is not clean and the return to the endmost position of the rotor causes a filing effect by friction of the tops of the threads of the stator against the ridges of the rotor, such friction generating wear and noise.
In a different techincal field, but closely related to the field of electric linear stepper motors, namely that of rotary electric stepper motors, numerous types of connection are known between the rotor, also driven with a cycloidal movement, and the output shaft of the motor which, for obvious reasons of convenience of use, must rotate on itself about a fixed shaft.
Thus the patent GB-A-1 246 444 describes different means for transmitting the rotational movement of the rotor to the output shaft, which means adapt themselves to the cycloidal movement of the rotor whereas the axis of the output shaft remains fixed. These means are for example universal joints, bellows or else excentric cams.
Similarly, U.S. Pat. No. 3,512,019 describes a flexible ring which transmits the movement of the rotor, whose axis here describes a cone, to the output shaft.
Furthermore, the application FR-A-2 492 605 describes resilient material disks which, in addition to their role of transmitting the movement of the rotor to the output shaft, have the function of maintaining, even at rest, a generatrix of the rotor in contact with a generatrix of the bore of the stator.
Finally, the French patent FR-A-1 317 985 describes an articulated resilient coupling which adapts itself to the cycloidal movement of the rotor, which cycloidal movement has a variable amplitude, for causing a variation of the rotational speed of the output shaft.
All these flexible or articulated connecting devices, whose function is to transmit the rotational movement of the rotor to the output shaft, must be indeformable under the action of a torsional stress, while being very deformable and very flexible under the action of a shearing stress. Their use in the linear motor of the above defined type would not overcome the drawback mentioned.